CALIFORNIA AGRICULTURAL EXTENSION SERVICE
CIRCULAR 133 FEBRUARY, 1947
A WEED KILLER
W. A. Harvey and W. W. Robblns
THE COLLEGE OF AGRICULTURE
UNIVERSITY OF CALIFORNIA • BERKELEY
SPECIES SUSCEPTIBLE TO 2,4-D
Austrian field cress*
Klamath weed (St. John's
Russian thistle (young)
Sow thistle (annual)
Star thistles (rosette)
White horse nettle*
* Those marked by an asterisk (*) may require two or more treatments.
Species Resistant to 2,4-D
2,4-D is Dangerous to Crops
Experiments to date show that of the crop plants, only the cereals and
other grasses are resistant to 2,4-D. Apparently most other crop plants
are injured by contact with the chemical, some of them seriously. For
this reason, the use of 2,4-D has so far been limited to control of weeds
in grain fields, grass pastures, and lawns, and to ridding soil of weeds
well in advance of planting crops.
As experiments with 2,4-D continue, changes are being made in
recommendations for its use. Consequently, the present recommenda-
tions must be regarded as tentative.
General Facts about 2,4-D
1. 2,4-D acts slowly and two weeks or a month may elapse before the
treated plants artually die.
2. Two or more sprayings are usually necessary to eflFect a complete kill of
many perennial weeds.
3. Temporary soil sterilization results from use of 2,4-D. How long soil
remains sterile depends upon amount of chemical used, temperature,
rainfall or irrigation, soil type, and crop planted.
4. Sprayers or other equipment used for 2,4-D must be thoroughly cleaned
before being used for other material.
5. Spray or dust must never be allowed to reach ornamental or susceptible
crop plants. Even small amounts of drift are highly injurious.
6. The acid 2,4-D and its compounds are nonpoisonous to humans and
animals, noncorrosive, and neither explosive nor inflammable.
2,4.D AS A WEED KILLER
W. A. HARVEY^ AND W. W. EOBBINS^
Certain so-called ' 'growth-regulating" compounds have found use as weed
killers. Chief of these is 2,4-D (or 2,4-dichlorophenoxyacetic acid). Weed
killers containing this chemical as the effective ingredient are known by many
trade names. At present over sixty commercial products containing 2,4-D are
registered with the Bureau of Chemistry, California State Department of
The earliest work with the material was carried on in the eastern United
States and in England. Extensive experiments and tests have been conducted
in California and elsewhere, and much more study is in progress. At present,
results are far from complete ; and recommendations for use are being con-
stantly revised as new information becomes available.
Although 2,4-D shows great promise as an economical herbicide, it has
sometimes been misused and some injury to crop and ornamental plants has
resulted. Yet 2,4-D, if used with care and discretion, will undoubtedly find
a permanent place in weed control.
Commercial Preparations of 2,4-D
The most common forms of 2,4-D are the acid (2,4-dichlorophenoxyacetic
acid) and the salts and esters of this acid. The term 2,4-D applies both to the
parent acid and its derivatives.
The Acid. The acid itself is a dry material almost insoluble in water. It is
usually mixed with a liquid carrier that will keep it in solution when diluted
with water. Such liquid preparations may differ in the amount of 2,4-D they
contain, and in the carrier used.
The Salts. Salts of the parent acid, such as sodium or ammonium salt, are
dry powders readily dissolved in water, and easily prepared. One dry material
now on the market contains 60 per cent of the sodium salt of the acid. A wet-
ting agent is also included in the preparation. This makes it possible to cover
the plants more evenly with the spray. Roughly, 1% pounds of a 60 per cent
sodium salt is equal to 1 gallon of liquid preparation having a 9.6 per cent
acid content. Several of the newer liquid preparations contain alkinolamine
salts, such as triethanolamine and others, which also wet leaves evenly.
The Esters. The esters used in the commercial 2,4-D products dissolve more
slowly in water than do the salts discussed above. They will, however, dissolve
easily in oil. For this reason, the esters are usually sold in an oil preparation
which mixes readily with water to form an emulsion. (The esters now available
in California are methyl, ethyl, and butyl, with isopropyl probably available
soon.) The content of 2,4-D in these newer preparations is 11 to 36 per cent,
depending upon the particular product. The number of pounds of 2,4-D per
gallon in any of the liquid preparations depends upon the strength of the
2,4-D (the per cent), and the weight per gallon. For this reason, the products
cannot be compared on the basis of per cent 2,4-D alone. Sometimes the label
^ Associate in Botany and Associate in the Experiment Station.
® Professor of Botany and Botanist in the Experiment Station.
4 California Agricultural Extension Circular 133
states the actual pounds or ounces of 2,4-D contained. This information helps
to determine the amount to use, since recommendations are usually made
in pounds of 2,4-D per acre.
Other preparations, some of which will undoubtedly contain higher per-
centages of 2,4-D, are likely to appear on the market.
So far, tests have shown that the various commercial 2,4-D products are
about equally effective when compared on the basis of the actual amount of
acid or acid derivative they contain. Thus in deciding which product to buy,
prices should be compared on the basis of 2,4-D content. Ease of handling
and mixing are other factors to be considered — a slightly faster mixing time
may compensate for a slightly higher price.
The acid 2,4-D and its compounds are nonpoisonous, noncorrosive, and
neither explosive nor inflammable.
How 2,4.D Kills Plants
This new chemical, which acts differently from other weed killers, was first
used in very weak concentration to regulate the growth of plants. Later, when
stronger concentrations were found to kill certain plants, its use as a weed
killer was developed.
Plants sprayed with 2,4-D react in various ways. The first noticeable effect
is in the stems and leaves, which twist and bend, the stems sometimes forming
loops and coils. In some plants, the stems and leaves dry until the tops are
completely dead; in others, the stems remain green for several weeks, but
may swell, develop cracks, and form callus tissue. Sometimes thick pads of
tissue develop along stems and at the joints. Often, numerous watery, trans-
lucent buds appear at the crown but do not grow into new shoots. Before
dropping, leaves of some woody plants change their color to red or yellow,
as though it were autumn. Several weeks after treatment, seriously affected
plants may show spongy, enlarged roots. The outer portion of the root may
slough off and leave wet, stringy cores that will later dry up or rot.
On most weeds, 2,4-D acts more slowly than other weed killers. It may
require from four to eight weeks for the weeds to die down completely. The
effects of spraying show up more rapidly in hot than in cool weather, but
the end result is the same.
Plants that form rosettes are especially susceptible in the rosette stage.
Other plants should be young and growing vigorously, with a well-developed
leaf surface. Old, mature plants respond slowly or not at all. All plants are
more easily killed as small seedlings, if application is made at that stage.
In general, broad-leaved plants are relatively susceptible to 2,4-D, but there
are exceptions. For example, it is usually easier to effect a permanent kill of
broad-leaved annuals than of broad-leaved perennials.
Effects of 2,4-D on Perennial Weeds
Among perennial weeds, these results have been observed in California :
Russian knapweed : Variable — per cent of kill ranges from 95 to 10. Should
be treated in tlie early rosette stage. Difficult to wet, and spray may require
a wetting agent.
Klamath weed: Early sprayings not too effective. Better results if plants are
treated when well developed, but before bloom stage.
2,4-D AS A Weed Killer
Fig. 1. — Injury of Tokay grapes from an application of 2,4-D in the vineyard for
the control of dogbane. Left, longitudinal section through old trunk shows develop-
ment of callus tissue ; right, new shoot shows distorted leaves.
Poison oak and wild blackberry: Not seriously affected by a single treatment
in early spring. Later applications, following complete leafing, look more
promising. Both plants classified as resistant.
Dandelion: Highly susceptible. One application usually kills entire infes-
Cattail, tule, bur-reed, and kelp: Proper applications effective even when
plants are rooted below the water surface. Cattails become resistant by early
summer. Three gallons of Diesel oil added to each 100 gallons of spray help
in penetrating the waxy surface of the plants. Ester preparations are espe-
cially effective on these species.
Water hyacinth, yellow water- weed, and hydrocotyle : These floating water
weeds are easily destroyed by spraying their above-water portions.
Nutgrass (which is not a true grass, but a sedge) : Somewhat susceptible.
Treatments must be repeated to destroy underground nuts where food is stored.
6 California Agricultural Extension Circular 133
In consulting lists (cover and p. 2), it would be well to bear in mind that
several factors influence the success of any treatment. Among those to be
considered are: susceptibility of the weed; stage of growth; amount of leaf
surface ; ease of wetting ; kind and strength of weed killer used ; and weather
conditions. With further field experience and additional tests, revision of the
list may be required.
Effects of 2,4-D on Grasses
Since members of the grass family are more resistant to 2,4-D than are
broad-leaved plants, the chemical is being widely used as a selective spray in
Barley and wheat : The usual rate of application has been % to % pound of
2,4-D per acre, applied in 100 to 200 gallons of water, with a ground rig.
Although some injury is noted when very young plants are treated, no dam-
age results when applications are made on grain 4 to 6 inches high. Wild
radish, mustards, fiddle-neck, and star thistle are readily killed in the young
stages by applications that ordinarily will not damage grain.
Corn and milo : One spraying before the crop plants cover the rows will greatly
reduce the infestation of such perennials as wild morning-glory and kelp for
the season. One to 1% pounds of 2,4-D per acre is the recommended amount.
Rice: In rice fields, airplane applications of 2,4-D have been successful in
control of arrowhead lily, water plantain, burhead, certain sedges, and other
water weeds. The usual treatment was 15 gallons per acre of a solution con-
taining 1 to 1% pounds of 2,4-D. (One pound is probably sufficient.) Where
the water was low when spray was applied, there was some damage, but fields
sprayed when the water was up and the plants well established showed no
Grass pastures, turfs, and grass seed fields : These areas may be treated with
1% pounds of 2,4-D per acre, in 100 to 200 gallons of water. If the grasses are
in the seedling stage, however, not more than % pound of the acid should be
used. Grass seed fields should not be treated when plants are in bloom. In
mixed pastures, 2,4-D will seriously injure or kill clovers, filaree, and other
broad-leaved forage plants.
Lawns : The correct solution is about %o ounce of the parent acid in 5 gal-
lons of water per 1,000 square feet of lawn (1^/2 pounds in 200 gallons of
water per acre). Treatment has been very effective in controlling dandelion,
plantain, chickweed, bur clover, green oxalis, pennywort, heal-all, mouse-ear
chickweed, and speedwell. Bluegrass and ryegrass are more resistant than
bent grasses and red top. The spray is not effective against Bermuda grass,
crab grass, and red oxalis — ^weeds common in bluegrass lawns.
Use of 2,4-D in Orchards
In orchards, 2,4-D has been used to control wild morning-glory and other
perennial weeds, except grasses. Thus far, no serious injury has been reported.
Drift has resulted in a slight amount of curling and discoloration of young
growth, but there was no stunting of tree growth or reduction in yield and
quality of fruit. If 2,4-D is used in orchards, however, care should be taken to
reduce drift to a minimum, and to avoid any undue accumulation of the
chemical in soil areas resulting from spilling or careless application.
2,4-D AS A Weed Killer
Effect of 2,4-D on Soil
In some instances, the use of 2,4-D has resulted in sterilization of the soil.
Some crops, such as broccoli, cabbage, sugar beets, tomatoes, beans, and root
crops have been damaged when grown in fields where 2,4-D had been used (see
fig. 2). However, in a number of cases, extra large amounts of the chemical
had been applied, and most of the fields
had remained dry from the time of ap-
plication until just before the crop was
Tests have shown that 2,4-D leaches
out of warm, moist soils in thirty to
sixty days, but it may remain in cool,
dry soils for six months or longer. Tests
also indicate that more 2,4-D is re-
tained by heavy than by light soils.
Flood irrigation after an application
of 2,4-D will help remove it, especially
in summer when the soil is warm. In
many areas, soil is safe for spring or
summer planting after the winter
rains. If areas are treated during the
late spring or summer, however, it is
not wise to plant susceptible crops un-
til several months have passed.
The Use of 2,4-D Dust
Tests to date indicate that while 2,4-D
applied as a dust is effective in killing
weeds, slightly more 2,4-D per acre
may be required than when applied as
a spray. Until some satisfactory method
is developed for control of drift toward
susceptible crop plants, it will be dan-
gerous to use dust near any crops ex-
Tests in which dry 2,4-D was applied to the soil at the time of seeding grains,
to destroy young germinating weed seedlings, have been disappointing. Grain
was injured much worse than by spray treatments. Success of dry applications
to the soil depends upon the amount of rainfall and, under California condi-
tions, cannot yet be recommended.
The Best Time to Spray
All available information indicates that weeds should be sprayed while still
young and growing vigorously. If plants are sprayed when they are old or
near maturity, the chemical will have a slow, uneven reaction. Although most
weeds should not be sprayed when temperature is low, certain early spring
weeds are making vigorous growth at this time, and applications of 2,4-D give
Fig. 2. — Eoot systems of onion bulbs:
The upper root system was grown in a solu-
tion containing 1 part per million 2,4-D
(.0001 per cent) ; the lower, in 5 parts per
million 2,4-D (.0005 per cent).
8 California Agricultural Extension Circular 133
Cost of Treatment
These weed-killing substances are usually applied as a spray. If an airplane
is used, only about 15 gallons per acre are required, as compared with 100 or
more gallons per acre when a ground rig is employed. The amount of 2,4-D
used depends upon the thickness of the weed growth, but ranges from % to
3 pounds per acre. The following figures are average costs per acre for some
of the commercial products: for selective weed control in grain, % pound
per acre, $2 to $3 ; for morning-glory, 1% pounds per acre, $4 to $6 ; and for
some of the more resistant weeds, $8 to $12. Prices may vary in different
localities, depending upon amounts available and quantity purchased.
In addition to the cost of the chemical there is the cost of application. This
is about $2 per acre, but varies with type of equipment and amount of acreage
sprayed. If a satisfactory method of applying dust is developed, the over-all
cost of treatment may be reduced.
Use 2,4-D with Caution
1. Do not expect miracles. 2,4-D is a new material, not thoroughly tested.
Its action is slow, sometimes requiring a month or longer to kill the tops and
roots of the weeds, especially perennials. Two sprayings may be necessary
because some plants are missed during the first spraying, and some new plants
may come up from lateral roots which did not die. Watch the sprayed area
closely and spray new growth or regrowth as soon as it is large enough.
2. Soil sterilization results from use of 2,4-D. How long the effects will re-
main depends upon amount of chemical used, temperature, rainfall or irriga-
tion, soil type, and crop planted. While grains and grasses apparently suffer
no damage if planted within a few weeks after spraying, beans, peas, lettuce,
tomatoes, cabbage, broccoli, sugar beets, alfalfa, and many other crops are
extremely sensitive to small quantities of the chemical.
3. A sprayer or any other equipment which has contained 2,4-D must be
thoroughly washed before it is used for other material. Otherwise, field,
orchard, and ornamental plants may be damaged if even a small amount
remains in the sprayer. One cold-water rinse is not sufficient. Use several
changes of water (preferably warm) to which a little baking soda or washing
soda has been added.
4. When spraying a lawn or other area, never allow the spray to reach
near-by ornamental or crop plants. Even small amounts of drift will injure
these plants, some of which are highly sensitive.
Recently, grapevines from the San Joaquin Valley have shown what ap-
peared to be damage from 2,4-D after treatment of morning-glory in the
vineyard. The effects on the vines may have been due to drifting of spray, to
action through soil, or both. Experience with it in vineyards is not extensive
enough, however, either to encourage or discourage its use for weed control
in such plantings. Other similar cases of injury to near-by crop plants by
spray have been reported. This past season, beans next to a rice field which
was sprayed from the air were injured by drift.